In the testing of underground tanks such as gasoline tanks for leaks, it has heretofore been considered essential that the temperature of the contents of the tank be monitored during the period of the test. The coefficient of expansion of gasoline is such that a change in temperature of 1.degree. F. of the contents of a 5000 gallon tank will cause a change in volume of about 3.25 gallons. Since the specifications of the National Fire Protection Association require that the rate of leaking from such a tank should not exceed 0.05 gallons per hour, it can be seen that if the test requires an hour or more to complete, the change in temperature during the test can give an erroneous result if this change is not taken into account. If the contents of the tank are decreasing in temperature, an erroneous indication of tank leaking may be given, whereas if the contents of the tank are increasing in temperature, the presence of an actual leak may be disguised by the increase in volume of the tank contents due to the temperature increase.
Another problem relating to temperature change is the fact that over a period of only an hour, the rate at which the temperature of the tank contents changes can vary substantially.
Various means for detecting such leaks and for compensating for the change in volume of the tank contents due to temperature change and evaporation during the test have been proposed. For example, U.S. Pat. No. 4,186,591 issued Feb. 5, 1980 shows a tank testing system having means for simultaneously monitoring the temperature and the level of the tank liquid. U.S. Pat. No. 4,300,388 issued Nov. 17, 1981 shows a tank testing device which utilizes a buoyant member suspended in the tank liquid, with means for compensating for changes in liquid level due to vaporization of the liquid and temperature changes. Obviously such compensation is necessary only if the test must be carried out over a considerable period of time. The above mentioned U.S. Pat. No. 4,300,388 utilizes a laboratory balance for supporting the buoyancy member to detect the change in level of the liquid, however it has been found that ordinary chemical or laboratory balances are not entirely satisfactory for such tank testing for a number of reasons.
In the case of a tank having a leak of the minimum amount required to be detected, (0.05 gallons per hour) the rate of change of the liquid level in the 4" fill pipe of the tank is only about 0.017 inches per minute. Although such scales are capable of accurately measuring small weight differences (which often requires that the beam be physically moved to the lower limit of its travel and then released to insure that the beam comes accurately to the balanced position), such scales are not suitable for measuring small increments of movement of a device suspended from the scale beam, since the scale is supported on a so-called knife edge. However such "knife-edge" does not actually provide an infinitely thin line on which the beam rests, but actually provides a narrow elongated surface having a finite width on which the beam pivots. This surface is also not absolutely regular, and therefore will cause a slight resistance to the starting motion of the beam, particularly when a very small force is applied very slowly and gradually to the beam. When the force reaches a sufficient amount, the beam will move suddenly, and will move slightly further than a distance which is proportional to the applied force.
This effect is sometimes referred to in the industry as "stiction", and has little consequence when the scale is used for measuring weight, since the beam can, if necessary, be moved by hand to a stop as described above, so that the force tending to restore the scales to the balanced position is relatively large. However, when the scale is used for detecting small changes in liquid level that occur over a period of time, with the beam position being accurately detected electronically, and with a chart recorder read-out of beam position, this erratic motion can cause wide fluctuations of the chart recorder pen which are considerably greater than the change due to liquid level change alone, and therefore require an extended test period to determine the "line of best fit " on the chart for actual rate of change of the liquid level.